The invention will be described in the environment of an enhanced definition television (EDTV) system, however it is not to be construed as limited to this application.
The television industry is striving to improve the quality of displayed television images. To this end several EDTV and high definition television (HDTV) systems have been proposed. The EDTV systems develop broadcast signals which are compatible for reception by existing standard receivers, but contain auxiliary signal components which may be utilized by EDTV receivers to generate enhanced images. The HDTV systems generate broadcast signals for producing high resolution, wide aspect ratio images on HDTV receivers, which signals are not compatible for reception by current day "standard" receivers. In both the EDTV and HDTV systems, in general it is necessary, either for regulatory reasons or pragmatic reasons to encode the original source image signals in a frequency spectrum of bandwidth narrower than the bandwidth of the source signals. Typically the encoding formats are established according to some statistically average signal feature whereby for the majority of transmitted images the respective receiver will be capable of faithfully reproducing the original image. However for certain images, for example the bandwidth of a particular encoded signal component may be insufficient and result in a poorer quality reproduced image. As an example consider the EDTV system described by Isnardi et al., entitled "Decoding Issues In the ACTV System", IEEE Transactions on Consumer Electronics, Vol. 34, No. 1, February 1988, pp. 111-120, also described in U.S. patent application Ser. No. 139,340, filed 29 December 1987 and incorporated herein by reference. The Isnardi et al. system develops an auxiliary signal component designated the vertical-temporal (V-T) helper signal, to aid the receiver in converting interlace scan signals to progressive scan signals.
The encoder of the Isnardi et al. system utilizes a progressive scan source of image signals and generates an interlace scan broadcast signal Nominally video signals contain significant information redundancy Due to this redundancy, receivers can be designed to autonomously convert interlace scan signals back to progressive scan signals fairly accurately. For images representing moving objects, the amount of redundancy diminishes and the ability of a receiver to autonomously convert interlace scan signals to progressive scan signals is impaired because the receiver lacks sufficient information. The V-T helper signal which has a variable amplitude provides this information. Since the helper signal represents only the receivers prediction error, it contains relatively low average energy for a majority of images. The bandwidth of the helper signal is limited to 750 .kappa.H2 to facilitate encoding, which bandwidth is sufficient to provide a helper signal with adequate information to reconstruct a majority of images. However the bandwidth is too narrow to provide sufficient helper information for images containing a high degree of detail and images which are panned. Consequently, the system performance may be deficient for a sequence of a certain class of images.